Irrigation Land-Leveling

Precision land-leveling, or precision grading, is the systematic process of removing soil from areas of higher elevation and depositing in areas of lower elevation in order to create a uniform slope. Precision grading is an approved soil and water conservation practice by the Natural Resources Conservation Service (NRCS standard 464) that is commonly implemented on the soils used for rice production in the Southeastern United States. A uniform slope for surface irrigation helps to increase irrigation efficiency and reduce water loss. Typically,land leveling will reshape the soil surface to a uniform drop of a tenth of a foot per hundred feet. Land-leveled fields usually have straight levees spaced 100 feet apart whereas non-leveled fields have levees that follow the contour and may be variably-spaced depending on the slope.

Precision grading provides many benefits for levee-based flood irrigation of rice and soybeans grown in rotation including: i) improved water management, ii) more efficient water use, iii) reduction of the number of levees required to maintain a given flood depth, iv) reduction of land area dedicated to levees and lost to optimum crop production, v) levees that are straight rather than curved to follow natural contour and vi) decreased risk of prolonged flooding of soybeans which can be detrimental to normal development and growth.

While land leveling offers many benefits, there also can be risks. Depending on elevation differences, the depth of soil removed, “cuts” can range from 0 inches to several feet. In many cases, part or all of the surface horizons can be removed, exposing sub-surface horizons. If the required soil removal is deep enough, sub-surface material may also be removed and deposited on top of surface soil. In many fields, the chemical, biological, and physical properties of the exposed and deposited sub-surface material render it unsuitable for profitable crop production. Corrective action is difficult, expensive and may require several years of treatment to achieve restoration. Growing green manures or treating freshly leveled ground with 2 tons per acre of raw poultry litter can help reduce these concerns.

Of particular concern is exposing soil horizons that contain excessive exchangeable sodium or soluble salts. Soils that naturally contain layers with elevated sodium levels at some depth in the profile comprise an estimated 494,000 acres in Arkansas and include the Lafe, Foley, and Hilleman soil series. Proceed with extreme caution and consult your local County Extension office or Conservation District office if land leveling these soils. In some cases, unaffected top soil can be stockpiled while leveling subsoil and replaced after leveling is complete to avoid exposing areas of high sodium or soluble salts.

Precision land leveling is a sizable investment that can cost hundreds to thousands of dollars per acre depending on the depth of cuts required. Typically, land leveling may require moving 100 to 600 cubic yards per acre at a cost ranging from $1.15 to a $1.50 per cubic yard. In 2013, the NRCS in Arkansas will cost-share land leveling at a rate of $1.40 per cubic yard.